Abstract: An electric guitar has an electromagnetic resonating unit (3) which is arranged to resonate the strings (2) of the guitar, either individually or together. The resonance of each string is picked up by a pick-up unit (4), and the frequency of resonation of the string is detected and compared with a desired value. A tension adjusting unit (5) adjusts the tension in each string (2) until the frequency of resonance of the string correponds to the desired frequency, in order to tune the guitar to a desired tuning pattern. A control panel (6) on the body (1) of the guitar displays optional tuning patterns which may be selected by a user. Automatic tuning of the guitar may be achieved, without requiring the strings to be plucked manually.

Abstract: A current sensor provides a positive voltage when a non-desired condition is present, such as rotation of machinery above a predetermined speed, and a gating circuit causes contacts of a relay to close whenever the rotational speed drops below such predetermined speed, that is when the desired condition is achieved. The gating circuit also closes these relay contacts in response to failure of the electrical power to such gating circuit. The circuit includes a P-N-P transistor in series with the relay coil, which transistor is switched on or off by a NAND gate. A low level signal to the base of the P-N-P transistor is provided through the NAND gate when the inputs to the gate are high, thereby providing a fail-safe feature for the relay because one of the NAND gate inputs is provided from the sensor's reference voltage through a Schmitt-trigger circuit.